This invention relates to a semiconductor circuit device including a diode arrangement as an overvoltage protection element, which is formed integrally with a power transistor for switching an inductance load such as an igniter for use in an engine ignition circuit, and sets a stable breakdown voltage to protect the transistor.
An IC semiconductor device includes a protection diode as one kind of an overvoltage protection element. The diode can be variously designed to lower the breakdown voltage V.sub.Z in comparison with the emitter-to-collector voltage V.sub.CEO of a transistor which is protected when the base of the transistor is open. For example, a portion of an inactive base region in the semiconductor substrate is deeply formed as a diffusion layer, and thus the breakdown voltage V.sub.Z is established by a reach-through into the high concentration collector region. The breakdown voltage V.sub.Z can also be created by implanting ions into a portion of an inactive base region to form a high concentration region having the same conductivity as that of the collector under high precision control. Such a semiconductor circuit device includes an active region therein, and thus a variation in the breakdown voltage V.sub.Z presents no problem. However, an extra manufacturing step is required to form a protection diode. Thus, a complicated manufacturing step is required in the fabrication of such a semiconductor circuit device. Now suppose that such a semiconductor circuit device is used for controlling an ignition coil mounted, for example, on an ignition control device of an internal conversion engine. It is, therefore, very difficult to set the breakdown voltage of 360 V.+-.10% necessary for controlling the ignition signal. The result is an expensive product which has a low yield.